1. Field of the Invention
The present invention relates generally to the field of high power light emitting diodes, and epitaxial films, and more particularly to high power gallium nitride on sapphire light emitting diodes, and to the transfer of GaN films from a growth substrate to a secondary substrate, and to vertical gallium nitride and while gallium nitride diodes.
2. Description of the Prior Art
Gallium nitride (GaN) based light emitting diodes (LEDs) are an important source of solid state lighting in the UV through Green portion of the spectrum. Unlike other LED material systems (e.g., indium phosphide, gallium arsenide), GaN is generally grown on high quality sapphire (Al2O3) or silicon carbide (SiC) substrates, since high quality GaN wafers are not commercially available. SiC is much more expensive than sapphire, and it is difficult to obtain high quality GaN films on SiC due to lower SiC quality as compared to sapphire. Consequently, sapphire is the best commercial choice of GaN epitaxy substrate.
Unfortunately, sapphire is a poor thermal conductor. This presents difficulties in removing heat from the LED chip while it is in an operating state. The higher the electrical input current, the more heat is generated in the chip, and the hotter the chip becomes, due to the limited thermal conductance of the sapphire substrate. This problem is greatly exacerbated with increasing LED chip size, since these larger chips required much higher input currents to operate. In fact, inefficient heat sinking ability is one of the limiting factors in large area LED brightness. Chip temperature significantly affects the device lifetime and stability. This is especially true for blue or UV chips which use phosphors (e.g., YAG) for re-radiating white spectrum light.
Current state-of-the-art large area GaN LEDs rely on sophisticated packaging techniques, such as flip-chip bonding, to address heat removal issues. These packaging methods both increase costs and potentially reduce yields. Furthermore, with ever increasing chip size and input power, these techniques may not be sufficient in removing enough thermal energy from the LED chip to keep operating temperatures stable and low.
GaN is both thermally and electrically insulating, a non-desirable trait. It is therefore useful to replace the sapphire substrate with a secondary substrate that is thermally and electrically conducting (e.g., various metals).
A group at the University of California, Berkeley, has developed a method of removing a GaN film from sapphire by irradiating the GaN through the sapphire with a 248 nm KrF excimer laser. Due to the bandgap energy of sapphire relative to the wavelength of the laser, the laser beam will pass through the sapphire with minimal absorption. However, the bandgap energy of GaN is such that most of the laser beam is absorbed in the GaN film at the sapphire-GaN interface. This absorbed energy is sufficient to decompose the GaN into gallium (Ga) and nitrogen (N) in a very thin region at the sapphire-GaN interface. Since the melting point of Ga is 30° C., the post-irradiated GaN film can be removed from the sapphire by heating above 30° C.
While lower laser beam energy densities will decompose GaN, the energy densities required to achieve high quality post-delamination GaN films are generally above 600 mJ/cm2. At such energy densities, the beam size is limited to approximately 2 cm2 due to power limitations on commercially available lasers. Current state-of-the-art GaN is grown on 50 mm diameter sapphire wafers, although this technology is migrating to large wafers (75 mm, 100 mm). With a maximum laser beam size of 2 cm2, a 50 mm GaN film would require more than 10 separate delamination cycles. Generally, the first delamination produces high quality films, but high stresses build up at the interface between delaminated and non-delaminated areas. These stresses can produce significant numbers of cracks in the GaN film during subsequent delamination cycles. Such cracks significantly decrease yields in subsequent processing of the GaN film.
One method of limiting the amount of cracking in the GaN layer during laser processing is to etch trenches into the GaN prior to laser processing, in order to provide some chip-to-chip isolation. If these trenches are not etched completely through the GaN, then the chip-to-chip isolation is not very effective, and cracks still propagate from chip to chip. If the trenches are etched completely through the GaN layer, then isolation is effective, but the support wafer bonding layer is exposed to the laser, thereby becoming weakened, leading to potential delamination of the support wafer prior to complete laser processing.
A white spectrum LED cannot be generated directly from GaN material, nor any other know semiconductor. The current method, therefore, for generating solid state white light consists of fabricating a blue or UV GaN LED chip and applying a variety of phosphors (e.g. Yitrium Aluminum Garnet—YAG) on top of the chip during packaging. The phosphor absorbs the blue or UV chip radiation, and re-radiates a white spectrum radiation.
The composition, thickness and uniformity of the phosphor layer are critical in determining the quality of the resulting white light, including such parameters as brightness, Color Rendering Index (CRI) and color temperature. This conventional approach of applying the phosphor during packaging significantly complicates the packaging process, and limits the use of solid state white light sources. Furthermore, over time the phosphor layer often interacts with the GaN chip, the wire bonds, or the epoxy used to encapsulate the LED package, resulting in degradation over time and reduced lifetimes.
GaN is generally grown on a very high quality sapphire (Al2O3) substrate, which is both thermally and electrically insulating, a non-desirable trait. It is therefore useful to replace the sapphire substrate with a secondary substrate that is thermally and electrically conducting (e.g., various metals).
A group at the University of California, Berkeley, has developed a method of removing a GaN film from sapphire by irradiating the GaN through the sapphire with a 248 nm KrF excimer laser. Prior to this laser lift off process, the Berkeley group permanently bonded a conducting wafer (namely Si) to the GaN side opposite the sapphire growth substrate (the p-doped surface of the GaN layer). This Si wafer provided support to the GaN layer during lift off, and took the role of conducting substrate after the lift off.
Generally, it is difficult to make good ohmic contact to the p-doped GaN layer, and the metallurgy available for such a contact is limited. Furthermore, in the Berkeley method described above, the metal layer bonding the Si wafer to the GaN layer has to be highly reflective as well for use in light emitting diode applications.